Frequency hopped/direct sequence spread spectrum (FH/DSS) time division multiple access (TDMA) data links (hereafter FH/DSS data links) require synchronization to operate correctly. Often, this synchronization is achieved in phases. In each phase a higher level of synchronization is achieved. The lowest level of synchronization, sometimes referred to “coarse synchronization”, refers to synchronization on the order of magnitude of one of the TDMA time slots that are shared by all users of the network. The coarse synchronization is typically followed by a fine synchronization stage. Frequently, the time required for coarse synchronization is the major contributor to the total synchronization time.
Coarse synchronization usually requires an initial estimate of time before synchronizing to the correct slot of the network. In currently available FH/DSS data links, time required to acquire coarse synchronization is on the same order of magnitude as the accuracy of the initial time estimate. For example, for an initial time accuracy of one minute σ, a radio would require on the order of one to five minutes to acquire coarse synchronization.
Previously available techniques for obtaining coarse synchronization involved tuning a correlation device for a specific direct sequence patterns and dwelling until a received bit sequence with a high correlation is obtained. This process requires a length of time that is longer than the standard deviation of the initial time uncertainty.
Another available method of obtaining coarse synchronization requires the transmitter to go into a transmit mode in which a trivial signal is transmitted for the purpose of gaining coarse synchronization. This method has the inherent vulnerability of being susceptible to jamming and spoofing, and requires cooperation by the transmitter. Thus, in data link systems operating in certain environments, this can be a highly undesirable solution.
Solutions for synchronization to a FH/DSS data link generally fall into two models, serial-search acquisition and matched-filter acquisition. See for example PRINCIPLES OF SECURE COMMUNICATION SYSTEMS, pp. 148-173 (Artech House 1992) by Don J. Torrieri; and SPREAD SPECTRUM COMMUNICATIONS HANDBOOK, Ch. 3 pp. 956-991 (McGraw-Hill 1994) by Simon et al. which are herein incorporated by reference in their entirety. Serial-search acquisition requires the received signal to be mixed against a local version of the frequency hopping pattern. The down converted signal is then low-pass filtered and integrated for a period. If the received energy exceeds a threshold, synchronization is declared. If not, the local frequency hopping pattern is shifted in time and the process starts over. For modern data links, serial-search acquisition is often too slow. Likewise, matched-filter acquisition as described in the Torrieri reference requires longer than the initial time uncertainty as discussed above.
Consequently, a faster method of performing coarse synchronization in a FH/DSS data link when a user has poor knowledge of time (minutes) is needed.